Fix unbox_value_or with explicit type or implicit hstring (#656)

Author: oldnewthing

strings/base_reference_produce.h

@@ -318,7 +318,7 @@ WINRT_EXPORT namespace winrt
         }
     }
 
-    template <typename T>
+    template <typename T = hstring, std::enable_if_t<std::is_same_v<T, hstring>, int> = 0>
     hstring unbox_value_or(Windows::Foundation::IInspectable const& value, param::hstring const& default_value)
     {
         if (value)

test/old_tests/UnitTests/Boxing2.cpp

@@ -214,4 +214,10 @@ TEST_CASE("Boxing")
         REQUIRE(unbox_value_or<UnsignedEnum>(box_value(static_cast<int32_t>(UnsignedEnum::Second)), UnsignedEnum::First) == UnsignedEnum::First);
         REQUIRE(unbox_value_or<UnsignedEnum>(box_value(static_cast<uint16_t>(UnsignedEnum::Second)), UnsignedEnum::First) == UnsignedEnum::First);
     }
+
+    {
+        // Test some cases where the compiler has to choose between multiple overloads.
+        REQUIRE(unbox_value_or<IInspectable>(nullptr, {}) == IInspectable{});
+        REQUIRE(unbox_value_or(nullptr, hstring{}) == hstring{});
+    }
 }